The process of updating a record in MySQL

stay MySQL in ,update Is to update data in place , Update data in place , Update data in place . Say the important things 3 All over . This is different from PGSQL Of .

update What is the specific process ：

(1)、 For this article record The corresponding index plus X lock

(2)、 Write the modified data to redo.log in

(3)、 Back up the data before modification to undo.log in

(4)、 Backup complete , Update data in place , modify txid, take roll_ptr Point to undo.log Backup of this record in , Last commit transaction tx

(5)、 Commit transaction tx In the process , The server went down suddenly , When restarting again , replay redo.log Log in to repair data .

If it has not been submitted before tx The server suddenly went down before , But the corresponding dirty page already flush To disk file , When the engine is started again , According to undo.log The logs in the are used for data recovery